FIG. 3 is a perspective view showing a conventional ink jet printer and FIG. 4 is a side view of the conventional ink jet printer. In FIGS. 3 and 4, a recording sheet fed by a non-illustrated pickup roller is nipped between a transport roller 4′ and a driven roller A′ and then moved a predetermined distance toward the left-hand side in the figure (in the secondary scanning direction). Subsequently, an ink head 12′ held by a carriage 2′ disposed downstream of the transport roller 4′ in the secondary scanning direction jets ink against the surface of the recording sheet while reciprocating in the primary scanning direction orthogonal to the secondary scanning direction. While intermittent feeding of the recording sheet in the secondary scanning direction and movement of the ink head 12′ in the primary scanning direction are repeated, an image forming operation continues. In response to detection of the trailing edge of the recording sheet just short of passing through the nip point of the transport roller 4′ by a non-illustrated sheet sensor, the ink head 12′ stops moving in the primary scanning direction. Finally, an ejection roller 10′ ejects the recording sheet out of the apparatus.
In this case, a blank of a distance X in which an image is not formed is produced in a trailing edge portion of the recording sheet, the distance X corresponding to the spacing between the nozzle position of the ink head 12′ and the nip point of the transport roller 4′. This results in a drawback that the image forming region of the recording sheet is limited in the trailing edge portion.
In recent years, some printers have been given the capability of forming an image on a recording sheet up to the trailing edge thereof without producing a blank. Mechanical measures devised for such apparatus to implement that function include: an arrangement configured to lower the sheet feed speed only for image formation on a portion of a recording sheet adjacent the sheet trailing edge; an arrangement made less susceptible to the influence of load fluctuations that occur at the moment the trailing edge of a recording sheet passes through the transport roller by increasing the sheet nipping pressure of an ejection roller section; an arrangement provided with a mechanism for suppressing backlash in driving the transport roller by a pressure produced by a spring; and an arrangement using parts, such as rollers, manufactured with an improved machining precision.
These apparatus, however, have a drawback of incurring complication of image formation control and increase in the costs of parts because they are also configured to perform high-quality image formation based mainly on the dot-jetting control technology. On the other hand, even in the case where attention is focused on the leading edge portion of a recording sheet under feeding in FIG. 4, load fluctuations occur at the moment the leading edge of the recording sheet rushes to an ejection driven roller 7′ which is configured to nip the recording sheet in cooperation with the ejection roller 5′ by exerting a pressing force on the ejection roller 5′, as in the case of the trailing edge portion. Such load fluctuations make the feed distance unstable, which results in degraded image quality such as color irregularity. A conventional ejection driven roller arrangement generally includes a plurality of such ejection driven rollers 7′ arranged in a row in the primary scanning direction. Accordingly, at the time the recording sheet rushes to the row of these ejection driven rollers 7′, pressing forces of all the ejection driven rollers 7′ are exerted on the leading edge of the recording sheet at a time, thus greatly affecting the sheet feed precision. It is needless to say that this phenomenon becomes more serious as the pressing force of each ejection driven roller 7′ increases.
In attempt to prevent irregular image formation at the trailing edge portion of a recording sheet, there have been proposed an arrangement wherein two types of materials, i.e., a soft material and a hard material, are used for the material of a first driven roller and two rows of such first driven rollers are arranged in the secondary scanning direction, and an arrangement wherein an auxiliary member for pressing against a recording sheet irrespective of the material and shape thereof is disposed downstream of the first driven roller (see patent document 1 for example). However, there is neither any mention of an arrangement of driven rollers of the same material in the primary scanning direction, nor any description of an arrangement of driven rollers pressing against ejection rollers. There has also been proposed an arrangement for a pressure control such as to release or substantially release the pressure of a driven roller pressing against a transport roller halfway through a printing operation (see patent documents 2 and 3 for example).
However, in a conventional ink jet printer which does not utilize the high-precision dot control technology, the amount of a feed or move of a recording sheet fluctuates due to fluctuations in the load on the sheet feed force which occur at the moment the trailing edge of the recording sheet passes through the transport roller, so that a line deviation occurs in the secondary scanning direction, thus causing a problem of degraded image quality such as color irregularity. Likewise, the sheet feed precision is affected at the time the leading edge of the recording sheet rushes to the ejection driven roller, thus resulting in degraded image quality such as color irregularity.
An object of the present invention is to provide an ink jet printer which can ensure improved image quality without incurring complication of image formation control and increase in cost.    Patent Document 1: Japanese Patent Laid-Open Publication No. H05-186086    Patent Document 2: Japanese Patent Laid-Open Publication No. H07-033279    Patent Document 3: Japanese Patent Laid-Open Publication No. H11-208923